conflict between system settings/network and entries in /etc/network/interfaces

[compdoc]

> as soon as I add an interface to /etc/network/interfaces the network
becomes inaccessible upon next boot

Be sure to check the entries in /etc/udev/rules.d/70-persistent-net.rules

This file adds new entries whenever a nic is added, but the system isn't
very good at removing entries if a card is removed. You can end up with
several entries that can screw up your networking. 

If there are two nics listed in yours, be sure the correct one is eth0. What
you should be able to do is delete all the entries and reboot, and the
system will repopulate 70-persistent-net.rules.

However, it doesn't always do a good job at that. Sometimes it works and
sometimes not. This way of handling network hardware really needs to be
tossed out.